Diagnostics

• If DI is suspected, sodium, plasma osmolality, and urine osmolality are tested (see expected lab values in the table below).

• A water deprivation test then allows DI to be differentiated from primary polydipsia.

• The patient's response to the administration of desmopressin, furthermore, distinguishes CDI from NDI.

• If CDI is diagnosed, a CT scan or MRI of the head should be conducted to rule out brain tumors (especially craniopharyngioma).

Water deprivation test

• After obtaining baseline lab values, patients stop drinking water for 2–3 hours before the first measurement

• After 2–3 hours without drinking water

  • Test urine volume and osmolality every hour

  • Test sodium and plasma osmolality every two hours

• Water deprivation continues until one of the following occurs:

  • Urine osmolality rises and reaches a normal value (> 600 mOsmol/kg) → DI ruled out and primary polydipsia confirmed

  • No change in urine osmolality despite a rising plasma osmolality (> 290 mOsmol/kg)

  • Plasma osmolality > 295–300 mOsmol/kg or sodium ≥ 145 meq/L

• In the latter two situations → administer desmopressin (a synthetic ADH analog)

  • Monitor urine osmolality testing every 30 minutes for 2 hours

    • In CDI: Urine osmolality rises after desmopressin administration (renal ADH receptors are intact).

    • In NDI: Urine osmolality remains low after desmopressin administration (defective renal ADH receptors).